Was one of the folks with no random reboots. (I got the phone the first day of availability. Day before Thanksgiving)
When this was first being reported, I was certain it was SD card related since I hadn't put an SD card in. But a few days ago, when the Facebook Beta app was released as non-beta and simply call Facebook, I installed it.
Within an hour of installing it, I noticed my 950XL was booting up just sitting at my desk, untouched. The ONLY thing I could think of that had changed was installing the FB app. So I uninstalled it and that was the end of that. No more reboots.
A couple of days ago I finally put a new Lexar SD card in. Still no reboots. (The latest model of 64gig. The $50 variety on Amazon)
Oddly though, on 3 occasions, the phone has displayed a message about the proper way to remove the SD card. It's as if it sensed the SD card being removed improperly. On each occasion, I immediately opened the File app and verified the SD card was accessible. And each time it was.
I currently have only pictures and music and offline maps designated for SD. All apps stay on system drive.
I honestly can't be sure if this is just software or just hardware. But given the fact that some units suffer dramatically and others none at all, it's not any clearer to me. I do find it curious that a few folks have even cured the problem. Sure makes it difficult to use process of elimination......the most common method of troubleshooting.
And finally, for those doing analysis, I have no issues with Band 2 or Groove in the truck. Nearly flawless Bluetooth behavior. In fact, other than the minor glitches I have described, this 950XL has been the best Lumia I've ever had. That's saying something since I'm a 920 and 1520 Lumiaista.